OF MONTE SOMMA AND VESUVIUS. 63 
é 
Remarks.—It will be convenient for the moment to consider the 
physical history of the ejectamenta we have been studying before 
returning to the stratigraphical part of the subject. 
Two salient facts are recognizable in the study of the series fee 
igneous products that have been included under the headings of 
Puases III. and IV., namely, that as we ascend from the lower to 
the higher members of the series, so do we find a more or less 
gradual and regular passage from an almost vitreous rock to one 
entirely crystalline in structure; this being nearly as regularly 
accompanied by the diminution of contained aqueous vapour, as 
represented by the diminished and more imperfect vesicular struc- 
ture of the upper members of these PHAszs. 
We have seen also that the break in time between one member 
and another was of comparatively short duration, and that the pro- 
ducts of later date are but the more highly developed crystalline 
condition of the magma from which was formed the white vitreous 
pumice. 
Reasons have already been given for believing that this white 
pumice shows itself to have been filled with aqueous matter under 
great pressure, and probably during a long period of time. On the 
contrary, the materials, following rapidly, would be exposed to 
hydration for a shorter time, as their stay in contact with the aqui- 
ferous strata would be limited. On arrival at or near the surface 
their ejection would be accompanied by less explosion, less rapid 
loss of heat, and consequently cooling would be slower, and there- 
fore more favourable to the development of the crystals and micro- 
liths constituting these rocks. 
Nevertheless, during the latter part of Paasz III. explosive action 
was still powerful enough to break the fluid magma into fragments, 
and it was not till all the highly aquiferous material had escaped 
that the quiet flow of lava could take place. 
There were undoubtedly fluctuations in the character and force 
of eruptive activity before the outflow of lava. The black scorize of 
Puase III., Period 2, show by their denseness, by the large quantity 
of foreign matter enveloped in the matrix, and by the rounded and 
broken character of the crystals, both natural and accidentally en- 
closed, that the fluid must have been for a long time exposed to 
violent ebullition and churning. In fact, we may well conceive the 
crater as filled by a fiery lake, which would be confirmed by the 
yellow altered lapilli in the bed above (Period 4); and that the lava 
fragments were rocks which had been submitted to fumarolic action, 
such as would exist around the crater-walls filled by a lava lake. 
Equally probably we may suppose a small cone of eruption to have 
formed in the crater of Period 1. 
Likewise we may explain those lateral openings and parasitic 
cones from which the intrapumiceous lava of Puasz IV. flowed. 
There is no evidence up to the present, so far as I can make out, of 
the lavas having flowed over the edge of the crater during this lava 
phase; but we must certainly require a column of fluid rock ex- 
tending up much higher than 700 metres (that of the highest lateral 
